I have a '97 5-speed with about 35k miles on it, mostly city miles. When the car was new, I would get a consistent 24-25 mpg. I can barely get 21 now and I am super-nice (2500 rpm upshifts, no a/c, etc). Also, the throttle response (still great compared to a FORD) has been much slower, almost stalled the car a few times due to this. Any idea what's going on here? Nothing has been replaced, just oil changes every 3750 miles.

 

Things to check ...
- low fuel pressure
- bad Throttle Position Sensor
- "lazy" Oxygen Sensors
- sticking fuel injectors
- dirty Throttle Body
- anything which prevents the Engine Control Unit from
going into closed loop mode (e.g. thermostat stuck open,
bad Engine Coolant Temperature Sensor)

 

How do you check/fix low fuel pressure?

Tryin to collect as much info as i can :>

 

Before you try any costly or intensive repairs, buy some throttle body cleaner and clean it out really good. It worked for me. I just went on a 400 mile trip and got 28 or 29 miles a gallon. Much improved over what I usually get.

 

Bleed the fuel rail pressure by pulling the fuel pump fuse and making several attempts to start the engine (it won't start). Install a T-fitting with a pressure gauge in the flexible rubber fuel line just downstream of the fuel filter. Install the fuel pump fuse. Start the engine. You should see the pressure maintain a stable 34 psi. After turning the engine off the fuel system should hold pressure for a long time, 24 hours or longer. If the residual pressure bleeds off in only one or two hours that is a sign of a problem... bad fuel pump check valve, injectors with an internal leak, bad fuel pressure regulator, or an external fuel leak. This is a standard diagnostic test. You will find instructions and a diagram or picture in Chilton (page 5-3) and Haynes (page 4-3,4).

96sleeper urged you to clean the Throttle Body before getting involved in more costly repair options. That's good advice. Along the same lines, you might replace your fuel filter as an early step. A partly clogged filter would reduce fuel pressure even if the pump is healthy. Inadequate fuel pressure results in a poor spray pattern at the injector tips. This might diminish the crisp throttle response you enjoyed when the car was new.

 

How about changing the spark plugs? Could this be a major contributor to my poor gas mileage? Or should they been in fairly good shape after 35k miles?

 

The factory plugs are good for 60K miles.

Did you replace the air filter element as part of your 30K service?

 

Yeah, I failed to mention that I put a K&N air filter in at 30k miles.

 

ill test that out sometime.. just bought some cleaner for the tb just the other day.. ive also had the fuel filter sitting in my garage forever.. gotta replace that..

 

Mine is a '96 SE with 175,000 miles on it, so we are in a little different situation. My Problem was my clutch. From what you described it sounds exacctly like what I experienced. My final clue that it was the clutch was when accelerating hard in gear at any rpm the engin would run up a few hundred rpm then drop down and then the power came in. I took it in to the dealer and got it replaced for $730. Now my mileage is back up, and my clutch is so tight I look like a new 5-spd driver hopping around in first. This might not be your problem, but I thought some advice can't hurt. Good luck!